wmaker: Cleaned dangerous function prototype usage

- remove extern declaration in source file, use header instead

- add inclusion of header defining the functions of the file to
get the compiler to cross-check them

- marked static the functions that should not be visible ouside
their file

src/properties.c

@@ -29,6 +29,7 @@
 #include "WindowMaker.h"
 #include "window.h"
 #include "GNUstep.h"
+#include "properties.h"
 
 /* atoms */
 extern Atom _XA_WM_STATE;